Classification of solid particles

Liquid cyclones can be used for the classification of solid particles over a size range from 5 to 100 jim. Commercial units are available in a wide range of materials of... 

Figure 49 shows a set of bed collapsing curves for a Geldart Group A-A (for Geldart s classification of solid particles, see Geldart, 1972, 1973) binary solids mixture, two closely sized alumina powders, of average particle diameter 104 and 66 microns, respectively. The curve on the extreme left with 0% fines represents the pure coarse component, which is... 

Liquid cyclones can be used for the classification of solid particles over a size range from 5 to 100 /rm. Commercial units are available in a wide range of materials of construction and sizes from as small as 10 mm to up to 30 m diameter. The separating efficiency of liquid cyclones depends on the particle size and density, and the density and viscosity of the liquid medium. 

Fig. 10.2 Flow regime map of gas-soUd contacting, a Characteristics of turbulent flow regime, b Characteristics of spouted beds, bubbling fluidized beds, fast fluidized beds and pneumatic transport regimes. In the figure notation the ordinate u = U p / n(pp — pg)g) is a dimensionless gas velocity, the abscissa d = dp[pg pp — Pg)glp ] a dimensionless particle size, the terminal velocity of a particle falling through the gas (m/s), and Umf the gas velocity at minimum fluidization (m/s). Letters A, B, C and D refer to the Geldart classification of solid particles. Reprinted from [49] with permission from Elsevier...

These operations may sometimes be better kno Ti as mist entrainment, decantation, dust collection, filtration, centrifugation, sedimentation, screening, classification, scrubbing, etc. They often involve handling relatively large quantities of one phase in order to collect or separate the other. Therefore the size of the equipment may become very large. For the sake of space and cost it is important that the equipment be specified and rated to Operate as efficiently as possible [9]. This subject will be limited here to the removal or separation of liquid or solid particles from a vapor or gas carrier stream (1. and 3. above) or separation of solid particles from a liquid (item 4j. Reference [56] is a helpful review. 

In solid-solid separation, the solids are separated into fractions according to size, density, shape, or other particle property (see Size REDUCTION). Sedimentation is also used for size separation, ie, classification of solids (see Separation, size separation). One of the simplest ways to remove the coarse or dense solids from a feed suspension is by sedimentation. Successive decantation in a batch system produces closely controlled size fractions of the product. Generally, however, particle classification by sedimentation does not give sharp separation (see Size measurement of particles). 

Particle Size Distribution. Almost every feed slurry is a mixture of fine and coarse particles. Performance depends on the frequency of distribution of particle size in the feed. Figure 5 shows that whereas all of the coarse particles having a diameter greater than some im are separated, fewer of the very fine particles are, at any given feed rate. The size distribution frequency of particles in feed and centrate for a fine and coarse feed are quite different. More coarse particles separate out than fine ones. Classification of solids by size is often done by centrifugal sedimentation. 

Separation in these devices known as winnowing machines [3], is achieved due to the difference between trajectories of coarse and fine particles in the separation zone (Fig. lb). Their operation and efficiency are strongly affected by the stochastic factors of the process, in particular by uncertainties in feeding and particles aerodynamic interactions. In most cases coarse particles prevent proper classification of fines. Separation efficiency of these devices is usually low. They are normally used for separation of solid particles according to densities (e.g. grain from peel), rather than by size. Sometimes crossflow separation in horizontal streams is used in combination with other separation principles. 

As is the case in most discussions of interfacial systems and their applications, definitions and nomenclature can play a significant role in the way the material is presented. The definition of an emulsion to be followed here is that they are heterogeneous mixtures of at least one immiscible liquid dispersed in another in the form of droplets, the diameters of which are, in general, greater than 0.1 (.m. Such systems possess a minimal stability, generally defined rather arbitrarily by the application of some relevant reference system such as time to phase separation or some related phenomenon. Stability may be, and usually is, enhanced by the inclusion of additives such as surfactants, finely divided solids, and polymers. Such a definition excludes foams and sols from classification as emulsions, although it is possible that systems prepared as emulsions may, at some subsequent time, become dispersions of solid particles or foams. 

Differential settling methods. The separation of solid particles into several size fractions based upon the settling velocities in a medium is called differential settling or classification. The density of the medium is less than that of either of the two substances to be separated. 

Classification of the separation techniques according to those involving phase change or mass transfer from one phase to another, known as diffusional operations, and those that are useful in the separation of solid particles or drops of a liquid and that are generally based in the application of an external physical force, known as mechanical separations. 

As the efficiency of separation is very often particle-size dependent, some separational equipment can be, and often is, also used for the classification of solids. This is the area where the grade efficiency concept was first developed it is now also widely used in gas cleaning. 

Gravitational settling of particles in liquids is an age-old process which can be used for a variety of purposes. For example, it is used for the classification of solids, washing, particle size measurement or mass transfer, and in solvent extraction. The majority of applications of gravity sedimentation, however, are in solid-liquid separation duty. The object here is to remove the solids from the liquid either because the solids and/or the liquid are valuable or because the two phases have to be separated before disposal. 

ABSTRACT. The classification of solid clathrate solutions may be subdivided into three types interstitial solutions, those with the substitution of one guest by another and those with the substitution of the particles in a host framework is given. All these types of solutions are illustrated by experimental (or computed) state diagrams of binary and ternary systems of guest-host and host-guestl-guest2 kinds, where host components are water, urea, thiourea and hydroquinone. 

It can be used for the classification of solids in liquid suspension, where a single cut i.s required between two si es of solid particle (or. less often, between solids of differing density). It is a very good device for this purpose, and its early history included development for the kaolin (china clay) industry. 

As far as possible, then, clarification aims at a complete separation of solids from the liquid stream. The next purpose, by contrast, aims specifically to leave some solids in the exit liquid. In the classification of solids by a decanter, a slurry of solid particles of mixed particle size, or, less often, of mixed densities, is treated in such a way that a specific fraction is removed as separated solid, leaving a well-defined fraction of the original solids still in suspension. This mode of operation is particularly relevant to the processing of kaolin (china clay), and it also finds a place where the decanter is used to remove oversize material, ahead of a more efficient clarifier, which might interfere with the final separator s operation (e.g. which might block the nozzles of a disc centrifuge). The decanter is a very efficient means of effecting classification by particle size. 

Fluidized Bed n An expanded bed of solid particles, fluidized to create a system with similar properties of a liquid. Fluidized beds are used successfully in a multitude of processes both catalytic and noncatalytic. Among the catalytic uses are hydrocarbon cracking and reforming, oxidation of naphthalene to phthalic anhydride, and ammoxidation of propylene to acrylonitrile. A few of the noncatalytic uses are coating of sulfide ores, coking of petroleum residues, drying, and classification. 

A particular focus of this chapter is colloidal dispersions of solid particles in a liquid. These are both industrially important but also scientifically interesting since model systems can be prepared with which we can probe the intermolecular interactions responsible for colloidal aggregation. As indicated in Table 3.1, such systems are termed sols. Sometimes they are also known as lyophobic solids. This reflects a now-outmoded classification of colloids into those that are solvent hating (lyophobic) and those that are solvent loving (lyophilic). Some examples of sols are described in Section 3.9, whilst the aggregation of model sols is discussed in Section 3.15. Other examples of commonly encountered colloids are described in Sections 3.10 to 3.14. 

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